We have shown recently that Mfn2 overexpression caused growth suppression in multiple cell lines by suppressing Ras-Raf-MEK-ERK signaling pathway. A 2-fold increase in cell proliferation was also observed in Mfn2-knockout murine embryonic fibroblast (MEF) cells as compared to wild type cells, and the proliferative advantage of the knockout MEF cells was blocked on reintroduction of the Mfn2 gene. We have demonstrated that Mfn2 exerts its anti-proliferative effect by acting as an effector molecule of Ras. Structure-function analysis studies reveal that the N-terminal fragment of Mfn2 (1-264) shares sequence similarity with the Ras-like GTPase superfamily members; whereas the C-terminal fragment of Mfn2 (265-757) shares a striking similarity with another Ras effector molecule and a tumor suppressor protein, NORE1A. Both the N-terminal and the C-terminal fragments of Mfn2 inhibit cell proliferation, and each acts by a distinct mechanism: the N-terminal mediated inhibition was due to its interaction with and inhibition of Raf-1, whereas the C-terminal fragment of Mfn2 inhibited cell proliferation by interaction with Ras; and the inhibition of proliferation by the N-terminal fragment is independent of its mitochondrial localization. Collectively, our data provide new insights regarding the role of Mfn2 in controlling cellular proliferation. To investigate the physiological relevance of the anti-proliferative properties of Mfn2, we extended our studies to human peripheral blood T cells. We wanted to know what happens to the Mfn2 levels during T cell activation through T cell receptors. Our data have demonstrated that the activation-induced degradation of Mfn2 preceded the cells entry into the cell cycle. Pharmacological inhibitors (LY294002, a PI3 kinase inhibitor; rapamycin, an mTOR inhibitor; and A443654, an AKT inhibitor) that blocked the activation-induced degradation of Mfn2, also blocked the cell cycle. The blockage in Mfn2 degradation was not a general consequence of cell cycle inhibition, since late cell cycle blockers (aphidicolin, an S-phase blocker; and Nocodazole, a G2/M blocker) blocked the cell cycle without preventing the degradation of Mfn2. These data suggest that the activation-induced Mfn2 degradation is a pre-requisite for cells entry into the cell cycle. It is known that T cells from aged mice have deficit in their proliferative capacities. To investigate the status of Mfn2 in the proliferative response of T cells from aged mice, naive and memory CD4+ T cells from young and old mice were activated through T cell receptor for 3 days, and the status of Mfn2 was determined by western blot analysis using whole cell lysates. The degree of activation-induced Mfn2 degradation was much less in memory populations as compared to nave populations, which correlated with the higher proliferative capacities of nave populations as compared to memory populations. We are currently investigating the mechanisms of activation-induced Mfn2 degradation.